In our experience the contribition of an experienced clinical pathologist was very useful at this step of diagnosis of mitochondrial disorders in children.51 Additional histological features such as lack of RRFs and normal fibre size proportion, presence of lipid accumulation of mild to remarkable degree and fibre size variability had only limited diagnostic value in differential diagnosis of LS associated with SURF1 gene mutations. This evidence concerns the gene SURF1 and Leigh syndrome.